Ultrasonic humidifiers are widely used in the household, office and hospital to moisturize the air to obtain good humidity range. In existing known technology, the ultrasonic humidifier controls its water level by one of the following methods: 1. the water tank is closed, and the opening is faced down; when the water level exceeds the opening, the water tank doesn't discharge water due to the barometric pressure so as to control the water level; 2. a water pump is used to deliver water to the working groove of the oscillating plate, the working groove is disposed with a gap; when in a high water level, water flows back to the water groove from the gap so as to control the water level. However, either the closing water tank or the water pump costs high that the products are not competitive.